1. Field of the Invention
The present invention relates to a drive circuit suitably applicable to a display device using, for example, an organic EL (Electro Luminescence) element. More particularly, the invention relates to a display device having the drive circuit.
2. Description of the Related Art
In recent years, in the field of a display device for displaying an image, a display device using, as a light emitting element of a pixel, an optical element of a current driving type whose light emission luminance changes according to the value of a flowing current, for example, an organic EL element is developed and is being commercialized. An organic EL element is a self-luminous element different from a liquid crystal element or the like. Consequently, in a display device using an organic EL element (organic EL display device), by controlling the value of current flowing in the organic EL element, tones of color are obtained.
Like a liquid crystal display, an organic EL display device has driving methods; a simple (passive) matrix method, and an active matrix method. The former method has, although the structure is simple, a disadvantage such that it is difficult to realize a large-size and high-resolution display device.
Consequently, at present, the active matrix method is actively developed. In the method, current flowing in a light emitting element disposed for each pixel is controlled by a drive transistor.
In the drive transistor, there is a case that a threshold voltage Vth and mobility μ changes with time, or varies among pixels due to variations in manufacturing processes. In the case where the threshold voltage Vth or mobility μ varies among pixels, the value of current flowing in the drive transistor varies among pixels. Consequently, even when the same voltage is applied to the gate of the drive transistor, the light emission luminance of the organic EL element varies, and uniformity of a screen deteriorates. Therefore, a display device having a function of correcting fluctuations in the threshold voltage Vth or mobility μ is developed (see, for example, Japanese Unexamined Patent Application Publication No. 2008-083272).
Correction on fluctuations in the threshold voltage Vth or mobility μ is performed by a pixel circuit disposed for each pixel. The pixel circuit includes, for example, as illustrated in FIG. 16, a drive transistor Tr1 for controlling current flowing in an organic EL element 111, a write transistor Tr2 for applying voltage of a signal line DTL to the drive transistor Tr1, and a retention capacitor Cs, and has a circuit configuration of 2Tr1C. The drive transistor Tr1 and the write transistor Tr2 are, for example, thin film transistors (TFTs) of the n-channel MOS type.
FIG. 15 shows an example of the waveforms of voltages applied to the pixel circuit and an example of changes in gate voltage and source voltage of the drive transistor. (A) in FIG. 15 illustrates a state where a signal voltage Vsig and an offset voltage Vofs are applied to the signal line DTL. (B) in FIG. 15 illustrates a state where a voltage Vdd for turning on the drive transistor and a voltage Vss for turning off the drive transistor are applied to a write line WSL. (C) in FIG. 15 illustrates a state where a high voltage VccH and a low voltage VccL are applied to a power line PSL. Further, (D) and (E) in FIG. 15 illustrate a state where a gate voltage Vg and a source voltage Vs change momentarily in accordance with the voltage application to the power line PSL, the signal line DTL, and the write line WSL.
It is understood from FIG. 15 that a WS pulse P1 is applied to the write line WSL twice in 1 H, threshold correction is performed by the WS pulse P1 of the first time, and the mobility correction and signal writing is performed by the WS pulse P1 of the second time. That is, in FIG. 15, the WS pulse P1 is used not only for signal writing but also for the threshold correction and the mobility correction in the drive transistor Tr1.
In the following, the threshold correction and the mobility correction in the drive transistor Tr1 will be described. By the application of the WS pulse P1 of the second time, the signal voltage Vsig is applied to the gate of the drive transistor Tr1. Accordingly, the drive transistor Tr1 is turned on, and current flows in the drive transistor Tr1. When it is assumed that a reverse bias is applied to an organic EL element 111, charges flowed from the drive transistor Tr1 are accumulated in the retention capacitor Cs and a device capacitor (not shown) of the organic EL element 111, and the source voltage Vs rises. In the case where the mobility of the drive transistor Tr1 is high, current flowing in the drive transistor Tr1 becomes large, so that the rise of the source voltage Vs is quickened. On the contrary, in the case where the mobility of the drive transistor Tr1 is low, current flowing in the drive transistor Tr1 becomes small, so that the rise of the source voltage Vs is slower than that in the case where the mobility of the drive transistor Tr1 is high. Therefore, by adjusting the period of correcting the mobility, the mobility is corrected.